A first background technology will be described. In products such as electromagnetic valves and motors, there has been a trend toward replacing electromagnetic steel plates with powder magnetic cores having superior magnetic characteristics over a wide frequency range. An example of a method for making this type of powder magnetic core is described in Japanese Laid-Open Patent Publication Number Hei 8-100203 (Patent Document 1), in which unsintered compact is made to form a metal composite part using powder metallurgy.
According to the method described in Patent Document 1, a slip additive is applied to the wall surfaces of a die electrostatically in the form of an aerosol of solid particles or liquid droplets. It would be preferable for the liquid droplets or solid particles to have a particle diameter of no more than 100 microns, more preferably no more than 50 microns, and even more preferably no more than 15 microns. Next, the die is filled with a metal powder composition, and this is compressed to form the unsintered compact. An unsintered compact with an especially high density is obtained when the compact made in this manner contains internal lubricant at a proportion of 0.1 percent by weight to 0.4 percent by weight, preferably 0.2 percent by weight to 0.3 percent by weight.
Also, Japanese Laid-Open Patent Publication Number Hei 9-104902 describes a powder compacting method that seeks to improve the material properties of a compact and the workability of the compact (Patent Document 2). In the powder compacting method described in Patent Document 2, a solid lubricant formed from a fatty acid or a metallic soap is sprayed onto a powder or the inner walls of a die before the die is filled with the powder. It would be preferable for the amount of sprayed solid lubricant to be 0.001 percent by weight to 2 percent by weight. For example, stearic acid could be sprayed onto the inner walls of a die at a proportion of 0.1 percent by weight.
A second background technology will be described. In electrical parts such as motor cores and transformer cores, there has been a demand for higher densities and more compact designs while allowing accurate control with low power. As a result, there has been active development of powder magnetic cores used to make these electrical parts that have superior magnetic characteristics especially in medium- and high-range frequencies. An example of a method for making this type of powder magnetic core is to add an organic lubricant to iron powder that has been surface treated to form a phosphate coating. The obtained mixed powder is compacted to form a compact. To remove distortions generated during the compacting, heat treatment is applied to the compact.
Also, Japanese Translation of PCT International Application Hei 6-507928 describes a magnetic powder composition used for magnetic parts and a method for making the same (Patent Document 3). The magnetic powder composition described in Patent Document 3 contains: iron powder coated with a thermoplastic resin; and a boron nitride powder mixed preferably at a proportion of 0.05 percent to 0.4 percent relative to the weight of the coated iron powder.
In the first background technology described above, Patent Document 1 and Patent Document 2 use a predetermined lubricant or solid lubricant to reduce friction during compacting. If a large amount of this lubricant is used, however, a non-magnetic layer takes up a high proportion of the powder magnetic core obtained by compacting, reducing the magnetic characteristics of the powder magnetic core. If a small amount of lubricant is used, lubrication during compacting is inadequate, causing the metal powders to rub against each other. Since this introduces significant distortion within the metal powders, the magnetic characteristics of the obtained powder magnetic core may be reduced. Also, if lubrication is inadequate during compacting, the die may not be filled with the metal powder in a uniform manner, or the density of the powder may be inadequate. This can lead to uneven or reduced density in the powder magnetic core.
Also, in the second background technology described above, a large amount of organic lubricant can be added to the iron powder coated with phosphate to prevent friction during compacting from destroying the phosphate coating. However, this increases the proportion of the organic lubricant in the powder magnetic core too much, leading to increased hysteresis loss in the obtained powder magnetic core. On the other hand, adding a very small amount of organic lubricant can limit the increase in hysteresis loss, but the phosphate coating will be destroyed during compacting, leading to increased eddy current loss in the powder magnetic core.
Also, since the organic lubricant has a relatively low thermal decomposition temperature, using a high temperature to treat the compact can lead to thermal decomposition of the organic lubricant and dispersion of the lubricant into the iron powder. This can lead to reduced magnetic characteristics for the obtained powder magnetic core. Furthermore, carbon (C) in the organic lubricant is left behind in the powder magnetic core as residue. Since carbon has a very low electrical resistance, it can lead to continuity between iron powders, thus increasing eddy current loss between particles in the powder magnetic core.
Also, if the powder magnetic core is used at a high temperature, the organic lubricant contained in the powder magnetic core may soften or melt. This will significantly reduce the strength of the powder magnetic core.
Also, the magnetic powder composition in Patent Document 3 contains boron nitride powder instead of an organic lubricant. However, since the proportion of boron nitride powder in Patent Document 3 is too high, the proportion of the magnetic body is small. This leads to reduced magnetic flux density of the magnetic powder composition and to increased iron loss from increased hysteresis loss.